Lighting fixtures have been used for many years in a variety of applications including theater and television lighting applications. Typically, each fixture includes an incandescent lamp mounted adjacent to a concave reflector, which reflects light through a lens assembly to project a beam of light toward a theater stage. A color filter can be mounted at the fixture's forward end, for transmitting only selected wavelengths of the light emitted by the lamp, while absorbing and/or reflecting other wavelengths. This provides the projected beam with a particular color composition.
The color filters used in these lighting fixtures typically have the form of glass or plastic films, carrying a dispersed chemical dye. The dyes transmit certain wavelengths of light, but absorb the other wavelengths. Several hundred different colors can be provided by such filters.
Although generally effective, such plastic color filters usually have limited lifetimes, caused principally by the need to dissipate large amounts of heat derived from the absorbed wavelengths. This has been a particular problem for filters transmitting blue and green wavelengths. Further, although the variety of colors that can be provided is large, these colors nevertheless are limited by the availability of commercial dyes and the compatibility of those dyes with the glass or plastic substrates. In addition, the mechanism of absorbing non-selected wavelengths is inherently inefficient. Substantial energy is lost to heat.
In some lighting applications, gas discharge lamps have been substituted for the incandescent lamps, and dichroic filters have been substituted for the color filters. Such dichroic filters typically have the form of a glass substrate carrying a multi-layer dichroic coating, which reflects certain wavelengths and transmits the remaining wavelengths. These alternative lighting fixtures generally have improved efficiency, and their dichroic filters are not subject to fading or other degradation caused by overheating. However, the dichroic filters offer only limited control of color, and the fixtures cannot replicate many of the complex colors created by the absorptive filters that have been accepted as industry standards.
It is sometimes necessary to change the color of the light being produced by a particular lighting fixture, so several remotely operated color-changing devices have been developed in recent years. One such device is a color scroller, which includes a scroll typically containing 16 preselected filters. These filters are subject to the same problems of fading and deformation as are the individual filters. Another such device is a dichroic color wheel, which includes a rotatable wheel carrying about eight preselected dichroic coatings. These color wheels avoid the noted problems of fading and deformation, but are able to carry fewer colors and are substantially more expensive than is a color scroller.
Other such remotely operated color-changing devices include a cyan, magenta & yellow (CMY) filter scroller system and a CMY dichroic color mixing system, the latter of which can provide about 16 million combinations of separate colors. However, because both CMY systems use filters that each transmit only about one third of the visible spectrum, they are unable to replicate the spectral nuances of a complex color, including those produced by a conventional color filter in combination with a full-spectrum incandescent light source.
Other remotely operated color-changing devices include an incandescent RGB fixture, such as a theatrical strip light. Such fixtures have similar problems to those of the two CMY systems described briefly above. In such fixtures, one-third of the visible spectrum is provided by each of three separately filtered lid sources. Thus, these fixtures waste two-thirds of the light energy just to project white light, and they waste even more light energy when projecting colored light.
Recently, some lighting fixtures have begun using light-emitting diodes (LEDs) instead of incandescent lamps and gas-discharge lamps. Equal quantities of red-, green-, and blue-colored LEDs typically have been used, arranged in a suitable array. Some LED fixtures have further included an equal quantity of amber-colored LEDs. By providing electrical power in selected amounts to these LEDs, typically using pulse-width modulated electrical current, light having a variety of colors can be projected. These fixtures eliminate the need for color filters, thereby improving on the efficiency of prior fixtures incorporating incandescent lamps or gas-discharge lamps.
Lighting fixtures incorporating red-, green-, and blue-colored LEDs, i.e., RGB LED fixtures, can project beams of light having an apparent color of white, especially when illuminating a white or other fully reflective surface. However, the actual spectrum of this apparent white color is not at all the same as that of the white light provided by fixtures incorporating incandescent lamps. This is because LEDs emit light in narrow wavelength bands, and merely three different LED colors are insufficient to cover the full visible spectrum. Colored objects illuminated by such RGB LED fixtures frequently do not appear in their true colors. For example, an object that reflects only yellow light, and thus that appears to be yellow when illuminated with white light, will appear black when illuminated with light having an apparent yellow color, produced by the red and green LEDs of an RGB LED fixture. Such fixtures, therefore, are considered to provide poor color rendition when illuminating a setting such as a theater stage, television set, building interior, or display window.
A limited number of LED lighting fixtures have included not only LEDs emitting red, green, and blue light, but also LEDs emitting amber light. Such fixtures are sometimes called RGBA LED fixtures. These fixtures are subject to the same drawbacks as are RGB LED fixtures, but to a slightly reduced degree.
Such LED lighting fixtures also generally require post-LED lens strategies for focusing. The optics are thus spaced apart from LED sources and thus render the system more expensive as well as require alignment of the optics to the light sources. Moreover, power levels attainable limit the applicability of the post LED lens approach. A new more efficient light concentration approach is needed to move the LED to the levels of output required for theater (theatrical illuminator) and other high intensity beam applications (such as automotive headlamps).